Radiotherapy is a process for the treatment of lesions such as cancers, which involves directing a beam of ionising radiation (typically high-energy x-rays) towards the lesion. Care is taken to maximise the dose that is applied to the lesion and minimise the dose that is applied to the areas of healthy tissue around the lesion, mainly by directing the beam toward the lesion from a multitude of directions and collimating the beam (i.e. shaping its cross-section) as required depending on the shape of the lesion. Various protocols exist for determining the beam directions, shapes, strengths and times to deliver a specific three-dimensional dose distribution to a specific region of the patient.
Clearly, it is necessary to ensure that the dose distribution is positioned correctly within the patient, i.e. that the patient and the radiotherapy apparatus are correctly. aligned. To do this, it is now common to take a cone-beam CT scan (“CBCT”) before treatment starts, and use that scan to compare the actual patient position with the treatment coordinate system to detect any mismatch. That is measured and calculated in 6 degrees of freedom, and the patient position is adjusted so as to match that which is expected by treatment coordinate system. For this purpose, patient table are known which are adjustable in 6 degrees of freedom, i.e. three translational directions and three rotational directions. This allows the patient to be brought into position with the apparatus, placing the location expected by the apparatus and at which the dose distribution will be delivered.